A biofilm may be described simply as “a community of microbes embedded in an organic polymer matrix, adhering to a surface” (Carpentier, 1993. J. Appl. Bacteriol. 75:499-511). All biofilms comprise three basic ingredients: microbes, a glycocalyx and a surface. If one of these components is removed, a biofilm will not develop.
A biofilm can be formed by a single bacterial species, but often biofilms consist of many species of bacteria together with fungi, algae, protozoa, debris and corrosion products. A biofilm can form on almost any surface exposed to bacteria and some amount of water.
The process of bacterial attachment to an available surface (living or abiotic) and the subsequent development of a biofilm is reviewed by W. Michael Dunne Jr, Clin Microbiol Rev. 2002 April; 15(2):155-66. Bacterial attachment is dictated by a number of variables, including the bacterial species, surface composition, environmental factors and essential gene products.
In simple terms, a biofilm forms when bacteria adhere to a surface in an aqueous environment and begin to excrete a slimy, glue-like substance which can anchor them to a range of materials—such as metals, plastics, soil particles, medical implant materials, and tissue.
Primary adhesion occurs through the chance meeting of a conditioned surface and a planktonic microorganism. As an oversimplification, primary adhesion between bacteria and abiotic surfaces is mediated by nonspecific (e.g. hydrophobic) interactions, whereas adhesion to living or devitalized tissue is accomplished through specific molecular (lectin, ligand, or adhesion) docking mechanisms. This stage is reversible and is dictated by physiochemical variables defining the interaction between the bacterial cell surface and the conditioned surface of interest.
After primary adhesion, an anchoring phase occurs, wherein loosely bound organisms consolidate adhesion by producing exopolysaccharides that complex with the surface, resulting in irreversible adhesion to the surface.
Once bacteria are irreversibly attached, biofilm maturation begins. The overall density and complexity of the biofilm increases, as surface-bound organisms actively replicate (and die) and extracellular components (generated by attached bacteria) interact with organic and inorganic molecules in the immediate environment to create the glycocalyx. Exopolysaccharides form the major component (excluding water) of the glycocalyx which, in most species, is predominantly anionic and traps nutrients while protecting the bacteria from environmental insults. In the case of infected biomedical implants, the glycocalyx may include host-derived inflammatory response proteins or matrix proteins such as complement, fibrinogen, and glycosaminoglycans attached to the implant.
The growth potential of a biofilm is limited by the availability of nutrients in the immediate environment, the perfusion of those nutrients to cells within the biofilm, and the removal of waste. An optimum hydrodynamic flow across the biofilm favours growth and perfusion rather than erosion of the outermost layers. Other factors that control biofilm maturation include internal pH, oxygen perfusion, carbon source, and osmolarity. At a critical mass, a dynamic equilibrium is reached at which the outermost layer of growth (farthest from the surface) generates planktonic organisms. These organisms are free to escape the biofilm and colonize other surfaces. Cells nearest the surface become quiescent or die due to a lack of nutrients or perfusion, decreased pH, pO2, or an accumulation of toxic metabolic by-products.
Once anchored to a surface, biofilm microorganisms carry out a variety of detrimental or beneficial reactions (by human standards), depending on the surrounding environmental conditions. Microbial biofilms on surfaces cost billions of dollars yearly in equipment damage, product contamination, energy losses and medical infections. Conventional methods of killing bacteria (such as antibiotics and disinfection) are often ineffective with biofilm bacteria, partially due to the protective nature of the glycocalyx. The huge doses of antimicrobials required to rid systems of biofilm bacteria are environmentally undesirable (and may not be allowed by environmental regulations) and medically impractical (since the amount required to kill the biofilm bacteria would also have an adverse effect on the patient).
Although surfaces or surface coatings that retard bacterial adhesion have been described (e.g. Sheng et al, Diagn. Microbiol. Infect. Dis. 38:1-5), none have been developed that prevent it (p1-11, Lappin-Scott and Costerton, Microbial Biofilms 1995. Cambridge University Press). Accordingly, new strategies are required to manage biofilm formation.